1. Field of the Invention
This invention relates to the field of video signal processing and, in particular, to the field of chroma keying circuits.
2. Background Art
Chroma keying circuits are required to detect very small changes in voltage and switch between video images and graphical images very quickly. In current state of the art equipment the switching rate required may be as high as forty-five megahertz. The voltage changes which are detected in these circuits in order to trigger a switch between video and graphical images may be only fifteen millivolts. Logic changes from zero to five volts on the output logic signal are often required. Thus the five volt switching signal is based upon a change in a video signal which is very fast and has a very small magnitude change. Sometimes this does not work properly creating distortions around the edges of images. Therefore it would be advantageous to develop a chroma keying signal that would more reliably key in on a video signal.
It is known in the art of switching between graphics signals and video signals to provide circuits which chroma key using blue. Thus, a video image against a blue background could be superimposed upon a graphics image by switching to the graphics image when the video image is blue and switching back to the video image when the video image is not blue. However, this prevented the use of blue within the video image. For example, a video image of a person with a tie having small amounts of blue may cause small portions of the graphics image to appear on the tie when the combined image is displayed. It has also been known for this to occur in images of people with blue eyes. Another problem with chroma keying on blue is that it is difficult to provide circuitry which can reliably and quickly distinguish between near blue and true blue.